How does this circuit keep a constant 20ma going to the LED

C

Chris W

Guest
I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs. Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf



--
Chris W
KE5GIX

"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm"

Ham Radio Repeater Database.
http://hrrdb.com
 
On Wed, 28 Nov 2007 01:03:17 -0600, Chris W wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs. Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf
Click to expand...
Maybe they're constant-current LEDs? Otherwise, there's no reason why the
circuit will maintain a constant current regardless of voltage.
 
Chris W wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs.
Click to expand...
It doesn't.

Graham
 
"Chris W" <1qazse4@cox.net> schreef in bericht
news:1p83j.3662$KK1.2400@newsfe24.lga...
I purchased a Ramsey Electronics BL1 kit. It is a very simple LED blinker
kit. All it does is alternately blink one led then the other. It seems
pretty obvious how it does the blinking. But the kit says it will work on
any where from 3V to 15V. What I can't understand, is how it can do that
and still provide the same 20ma to the LEDs. Or maybe it doesn't do that
but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf



--
Chris W
KE5GIX

"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm"

Ham Radio Repeater Database.
http://hrrdb.com
Click to expand...

You left out the most important component value: The LEDs. Unless they're
special types, the current through a LED will rise to about 130mA at 15V.
That's high, way too high for an ordinary 20mA LED. Even if you look only to
the average, so 65mA, it's too high. If you don't blow them up at once, they
will live short at that high a current. Now there are numerous
extra-ordinary LEDs these days. Types that can handle over 100mA. But I
don't expect that types to appear in a simple, lowcost kit like the one at
hand seems to be.

petrus bitbyter
 
On Wed, 28 Nov 2007 01:03:17 -0600, Chris W <1qazse4@cox.net> wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs. Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf
Click to expand...
As petrus has already pointed out the current would be too high at 15
volts with red LEDs. I figure (allowing for voltage drops in the leds
and transistors) you're still pushing ~120 ma through them with a 15 V
supply. The thing sits there toggling the leds off and on so the duty
cycle is 50% but that still means ~60 ma of average current.

R1 and R2 should be higher if you run at 15 volts. I'd use about 325
with a red led and 275 with white at 15 volts input. That should
limit current to around 40 milliamps and with 50% duty 20 milliamps
per led.

putting a white or blue led in there will work at 15 but not at 3
volts

The fact that they include R1 and R2 suggest that the leds aren't
self-limiting types - and the ones that limit current cost more, and
may preclude operating down at 3 volts.

Leds don't just "up and die" when you over-current them. They have a
half-life. Number of hours to half the brightness of the original
output. It is related to the temperature at the die - so ambient
temperature, heat sinking and current will all effect light output.

Led life expectancy specifications don't always reflect the truth -
yeah it will last 200,000 hours - but will only put out a fraction of
the light it is (was) rated at.

If you only expect it to be on for short periods at a time and need
the light output run it hot - if it will stay on round the clock -
reduce the power below 20 ma.

My first LED night light had three red leds in it and they were very
dim at the end of five years. I replaced the leds two years ago and it
is back to normal.
--
---- Posted via Pronews.com - Premium Corporate Usenet News Provider ----
http://www.pronews.com offers corporate packages that have access to 100,000+ newsgroups
 
On Wed, 28 Nov 2007 01:03:17 -0600, Chris W <1qazse4@cox.net> wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs.
Click to expand...
---
It doesn't.
---

Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?
Click to expand...
---
Sure, but why not get rid of the resistors altogether and feed the
blinker from a constant current source, like this:

Version 4
SHEET 1 880 708
WIRE -928 -144 -1088 -144
WIRE -640 -144 -928 -144
WIRE -496 -144 -640 -144
WIRE -160 -144 -496 -144
WIRE -16 -144 -160 -144
WIRE -640 -96 -640 -144
WIRE -496 -96 -496 -144
WIRE -160 -96 -160 -144
WIRE -16 -96 -16 -144
WIRE -640 32 -640 -32
WIRE -368 32 -640 32
WIRE -16 32 -16 -32
WIRE -16 32 -288 32
WIRE -928 80 -928 -144
WIRE -640 80 -640 32
WIRE -16 80 -16 32
WIRE -496 128 -496 -16
WIRE -496 128 -576 128
WIRE -464 128 -496 128
WIRE -368 128 -288 32
WIRE -368 128 -400 128
WIRE -288 128 -368 32
WIRE -256 128 -288 128
WIRE -160 128 -160 -16
WIRE -160 128 -192 128
WIRE -80 128 -160 128
WIRE -640 256 -640 176
WIRE -640 256 -832 256
WIRE -16 256 -16 176
WIRE -16 256 -640 256
WIRE -928 304 -928 160
WIRE -896 304 -928 304
WIRE -800 352 -832 352
WIRE -1088 400 -1088 -144
WIRE -928 400 -928 304
WIRE -896 400 -928 400
WIRE -800 448 -800 352
WIRE -800 448 -832 448
WIRE -896 496 -928 496
WIRE -800 496 -800 448
WIRE -1088 624 -1088 480
WIRE -928 624 -928 496
WIRE -928 624 -1088 624
WIRE -800 624 -800 576
WIRE -800 624 -928 624
WIRE -1088 688 -1088 624
FLAG -1088 688 0
SYMBOL npn -80 80 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn -576 80 M0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res -512 -112 R0
SYMATTR InstName R3
SYMATTR Value 100k
SYMBOL res -176 -112 R0
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL cap -400 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 10e-6
SYMBOL cap -192 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 10e-6
SYMBOL npn -896 256 R0
SYMATTR InstName Q3
SYMATTR Value 2N3904
SYMBOL npn -832 400 M0
SYMATTR InstName Q4
SYMATTR Value 2N3904
SYMBOL res -944 64 R0
WINDOW 0 -42 40 Left 0
WINDOW 3 -42 70 Left 0
SYMATTR InstName R5
SYMATTR Value 10k
SYMBOL res -816 480 R0
SYMATTR InstName R6
SYMATTR Value 30
SYMBOL voltage -1088 384 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value 6
SYMATTR InstName V2
SYMBOL LED -656 -96 R0
WINDOW 0 -29 -1 Left 0
WINDOW 3 -135 70 Left 0
SYMATTR InstName D1
SYMATTR Value NSPW500BS
SYMBOL LED -32 -96 R0
SYMATTR InstName D2
SYMATTR Value NSPW500BS
TEXT -1072 656 Left 0 !.tran 0 3 2 uic


To vary the current change the value of R6. Decreasing the value
increases the LED current.

For higher-voltage LEDs, increase the supply voltage to what's
required to get the current you want. Also, remember that what
voltage isn't dropped across the LEDs will be dropped mostly across
Q4, at whatever current is in the LEDs, so you may want to watch
Q4's dissipation.


--
JF
 
On Wed, 28 Nov 2007 10:16:19 -0600, John Fields
<jfields@austininstruments.com> wrote:

On Wed, 28 Nov 2007 01:03:17 -0600, Chris W <1qazse4@cox.net> wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs.

---
It doesn't.
---

Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

---
Sure, but why not get rid of the resistors altogether and feed the
blinker from a constant current source, like this:
Click to expand...
<snip>

---
Even simpler, since the two LEDs are driven out of phase with each
other, you could get rid of one resistor and do this:

Version 4
SHEET 1 880 708
WIRE -800 -224 -896 -224
WIRE -640 -224 -720 -224
WIRE -16 -224 -640 -224
WIRE -896 -144 -896 -224
WIRE -496 -144 -896 -144
WIRE -160 -144 -496 -144
WIRE -640 -96 -640 -224
WIRE -496 -96 -496 -144
WIRE -160 -96 -160 -144
WIRE -16 -96 -16 -224
WIRE -640 32 -640 -32
WIRE -368 32 -640 32
WIRE -16 32 -16 -32
WIRE -16 32 -288 32
WIRE -896 80 -896 -144
WIRE -640 80 -640 32
WIRE -16 80 -16 32
WIRE -496 128 -496 -16
WIRE -496 128 -576 128
WIRE -464 128 -496 128
WIRE -368 128 -288 32
WIRE -368 128 -400 128
WIRE -288 128 -368 32
WIRE -256 128 -288 128
WIRE -160 128 -160 -16
WIRE -160 128 -192 128
WIRE -80 128 -160 128
WIRE -896 256 -896 160
WIRE -640 256 -640 176
WIRE -640 256 -896 256
WIRE -16 256 -16 176
WIRE -16 256 -640 256
WIRE -896 320 -896 256
FLAG -896 320 0
SYMBOL npn -80 80 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn -576 80 M0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res -512 -112 R0
SYMATTR InstName R3
SYMATTR Value 100k
SYMBOL res -176 -112 R0
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL cap -400 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 10e-6
SYMBOL cap -192 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 10e-6
SYMBOL voltage -896 64 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value 15
SYMATTR InstName V2
SYMBOL LED -656 -96 R0
WINDOW 0 -29 -1 Left 0
WINDOW 3 -135 70 Left 0
SYMATTR InstName D1
SYMATTR Value NSPW500BS
SYMBOL LED -32 -96 R0
SYMATTR InstName D2
SYMATTR Value NSPW500BS
SYMBOL res -704 -240 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 620
TEXT -616 288 Left 0 !.tran 0 3 2 uic


--
JF
 
John Fields wrote:

Sure, but why not get rid of the resistors altogether and feed the
blinker from a constant current source, like this:

Version 4
SHEET 1 880 708
WIRE -928 -144 -1088 -144
WIRE -640 -144 -928 -144
WIRE -496 -144 -640 -144
WIRE -160 -144 -496 -144
WIRE -16 -144 -160 -144
WIRE -640 -96 -640 -144
WIRE -496 -96 -496 -144
WIRE -160 -96 -160 -144
. . .
Click to expand...
What program can I use to import those text instructions? I am using
the free ware version of Eagle to create schematics, and I can't see how
or if it supports a file with that format.


--
Chris W
KE5GIX

"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm"

Ham Radio Repeater Database.
http://hrrdb.com
 
On Wed, 28 Nov 2007 11:10:18 -0600, Chris W <1qazse4@cox.net> wrote:

John Fields wrote:

Sure, but why not get rid of the resistors altogether and feed the
blinker from a constant current source, like this:

Version 4
SHEET 1 880 708
WIRE -928 -144 -1088 -144
WIRE -640 -144 -928 -144
WIRE -496 -144 -640 -144
WIRE -160 -144 -496 -144
WIRE -16 -144 -160 -144
WIRE -640 -96 -640 -144
WIRE -496 -96 -496 -144
WIRE -160 -96 -160 -144
. . .

What program can I use to import those text instructions? I am using
the free ware version of Eagle to create schematics, and I can't see how
or if it supports a file with that format.
Click to expand...
---
LTSPICE, free at:

http://www.linear.com/designtools/software/



--
JF
 
On Wed, 28 Nov 2007 11:10:18 -0600 in sci.electronics.basics, Chris
W <1qazse4@cox.net> wrote,
What program can I use to import those text instructions? I am using
the free ware version of Eagle to create schematics, and I can't see how
or if it supports a file with that format.
Click to expand...
LTspice, free from http://www.linear.com/designtools/software/
 
John Fields wrote:
---
LTSPICE, free at:

http://www.linear.com/designtools/software/
Click to expand...

Thanks for the schematic and software link. I found how I can set it up
to graph a voltage vs current for each component, is there a way I can
graph voltage and or current vs time to see how fast the capacitor
charges and switches between LEDs?

I plan on running this at a constant voltage and will probably want to
add more LEDs to get more light, how much current can I run through
those transistors and still be safe? Five blue LEDs would be the most I
would use. I could also run them in series and just up the voltage I
guess, but about 12V is the highest I have a handy power supply for.


--
Chris W
KE5GIX

"Protect your digital freedom and privacy, eliminate DRM,
learn more at http://www.defectivebydesign.org/what_is_drm"

Ham Radio Repeater Database.
http://hrrdb.com
 
On Wed, 28 Nov 2007 09:41:37 -0500, default wrote:

R3 and R4 are 47k ohms
C1 and C2 are 4.7uf

R1 and R2 should be higher if you run at 15 volts. I'd use about 325
with a red led and 275 with white at 15 volts input. That should
limit current to around 40 milliamps and with 50% duty 20 milliamps
per led.
Click to expand...
RC = 47e3 * 4.7e-6 ~= 220ms.

I wouldn't assume that you can rely upon the "50% duty" part at such a low
frequency.
 
On Wed, 28 Nov 2007 15:11:05 -0600, Chris W <1qazse4@cox.net> wrote:

John Fields wrote:
---
LTSPICE, free at:

http://www.linear.com/designtools/software/





Thanks for the schematic and software link. I found how I can set it up
to graph a voltage vs current for each component, is there a way I can
graph voltage and or current vs time to see how fast the capacitor
charges and switches between LEDs?
Click to expand...
---
Right click on the schematic, then left click "Edit Simulation Cmd."
Next, click the 'Transient' tab, set the run time for what you want
and click 'OK'. You'll then be back in the schematic and you'll
notice a cursor with a little box attached to it that moves with
your mouse. Place it where you want to and left click your mouse and
you'll notice that it changes to text.

Now, right click and then left click 'Run' and you'll see that as
you move the cursor around in the schematic pane a little voltag
probe will appear whenever the cursor touches a wire and a little
current probe will appear whenever the cursor touches a component.

Left click the voltage probe and a voltage VS time waveform will be
plotted, left click the current probe and current VS time will be
plotted.
---

I plan on running this at a constant voltage and will probably want to
add more LEDs to get more light, how much current can I run through
those transistors and still be safe?
Click to expand...
---
http://www.fairchildsemi.com/ds/2N%2F2N3904.pdf
---

Five blue LEDs would be the most I
would use. I could also run them in series and just up the voltage I
guess, but about 12V is the highest I have a handy power supply for.
Click to expand...
---
Run the simulator with various schenarios to see what works for you.


--
JF
 
Chris W wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs. Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf



you assumptions are correct, the current will vary.
Click to expand...

--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5
 
On Wed, 28 Nov 2007 18:41:28 -0500, Jamie
<jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:

Chris W wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs. Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf



you assumptions are correct, the current will vary.
Click to expand...
---
Hmm...

Echoes are always a little late. ;)


--
JF
 
John Fields wrote:

On Wed, 28 Nov 2007 18:41:28 -0500, Jamie
jamie_ka1lpa_not_valid_after_ka1lpa_@charter.net> wrote:


Chris W wrote:


I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs. Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf




you assumptions are correct, the current will vary.


---
Hmm...

Echoes are always a little late. ;)


John, you're a great philosopher :)
Click to expand...


--
"I'd rather have a bottle in front of me than a frontal lobotomy"
http://webpages.charter.net/jamie_5
 
now we got a whole new language PSPICE! to deal with, this is like living
in outer monvoltia!


"John Fields" <jfields@austininstruments.com> wrote in message
news:p66rk3pqbg5u142h7m3rht2me5b4hvipql@4ax.com...
On Wed, 28 Nov 2007 10:16:19 -0600, John Fields
jfields@austininstruments.com> wrote:

On Wed, 28 Nov 2007 01:03:17 -0600, Chris W <1qazse4@cox.net> wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs.

---
It doesn't.
---

Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

---
Sure, but why not get rid of the resistors altogether and feed the
blinker from a constant current source, like this:

snip

---
Even simpler, since the two LEDs are driven out of phase with each
other, you could get rid of one resistor and do this:

Version 4
SHEET 1 880 708
WIRE -800 -224 -896 -224
WIRE -640 -224 -720 -224
WIRE -16 -224 -640 -224
WIRE -896 -144 -896 -224
WIRE -496 -144 -896 -144
WIRE -160 -144 -496 -144
WIRE -640 -96 -640 -224
WIRE -496 -96 -496 -144
WIRE -160 -96 -160 -144
WIRE -16 -96 -16 -224
WIRE -640 32 -640 -32
WIRE -368 32 -640 32
WIRE -16 32 -16 -32
WIRE -16 32 -288 32
WIRE -896 80 -896 -144
WIRE -640 80 -640 32
WIRE -16 80 -16 32
WIRE -496 128 -496 -16
WIRE -496 128 -576 128
WIRE -464 128 -496 128
WIRE -368 128 -288 32
WIRE -368 128 -400 128
WIRE -288 128 -368 32
WIRE -256 128 -288 128
WIRE -160 128 -160 -16
WIRE -160 128 -192 128
WIRE -80 128 -160 128
WIRE -896 256 -896 160
WIRE -640 256 -640 176
WIRE -640 256 -896 256
WIRE -16 256 -16 176
WIRE -16 256 -640 256
WIRE -896 320 -896 256
FLAG -896 320 0
SYMBOL npn -80 80 R0
SYMATTR InstName Q1
SYMATTR Value 2N3904
SYMBOL npn -576 80 M0
SYMATTR InstName Q2
SYMATTR Value 2N3904
SYMBOL res -512 -112 R0
SYMATTR InstName R3
SYMATTR Value 100k
SYMBOL res -176 -112 R0
SYMATTR InstName R4
SYMATTR Value 100k
SYMBOL cap -400 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C1
SYMATTR Value 10e-6
SYMBOL cap -192 112 R90
WINDOW 0 0 32 VBottom 0
WINDOW 3 32 32 VTop 0
SYMATTR InstName C2
SYMATTR Value 10e-6
SYMBOL voltage -896 64 R0
WINDOW 3 24 104 Invisible 0
WINDOW 123 0 0 Left 0
WINDOW 39 0 0 Left 0
SYMATTR Value 15
SYMATTR InstName V2
SYMBOL LED -656 -96 R0
WINDOW 0 -29 -1 Left 0
WINDOW 3 -135 70 Left 0
SYMATTR InstName D1
SYMATTR Value NSPW500BS
SYMBOL LED -32 -96 R0
SYMATTR InstName D2
SYMATTR Value NSPW500BS
SYMBOL res -704 -240 R90
WINDOW 0 0 56 VBottom 0
WINDOW 3 32 56 VTop 0
SYMATTR InstName R1
SYMATTR Value 620
TEXT -616 288 Left 0 !.tran 0 3 2 uic


--
JF
Click to expand...
 
From:

http://groups.google.com/support/bin/answer.py?answer=12348&topic=250

"Summarize what you're following up.

When you click "Reply" under "show options" to follow up an existing
article, Google Groups includes the full article in quotes, with the
cursor at the top of the article. Tempting though it is to just
start
typing your message, please STOP and do two things first.
Look at the quoted text and remove parts that are irrelevant.
Then, go to the BOTTOM of the article and start typing there.
Doing this makes it much easier for your readers to get through your
post. They'll have a reminder of the relevant text before your
comment, but won't have to re-read the entire article.
And if your reply appears on a site before the original article
does,
they'll get the gist of what you're talking about."


--
JF
 
Chris W wrote:
I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs.
Click to expand...

Others have already answered - I'll address what may
be an underlying assumption: Do you think 20 mA is
"sacred" for LEDs ? It isn't.


20 mA is a "rule of thumb" kind of number for typical
LEDs, but they will glow at a lot lower current. When
you start to go above 20 mA, you may go out of the "safe"
zone and cook them. And, there are a wide variety of
LEDs available, so it is best to read the datasheet for
the LEDs you intend to use.

Also, the current does not have to be constant for the LED
to glow. If it varies a lot, the brightness of the LED will
vary visibly which can be annoying. Less variation will be
less annoying. Again, if it goes too high you can cook the
LEDs.

Ed

Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf
Click to expand...
 
"Nobody" <nobody@nowhere.com> wrote in message
news:pan.2007.11.28.07.48.52.512311@nowhere.com...
On Wed, 28 Nov 2007 01:03:17 -0600, Chris W wrote:

I purchased a Ramsey Electronics BL1 kit. It is a very simple LED
blinker kit. All it does is alternately blink one led then the other.
It seems pretty obvious how it does the blinking. But the kit says it
will work on any where from 3V to 15V. What I can't understand, is how
it can do that and still provide the same 20ma to the LEDs. Or maybe it
doesn't do that but the book sure makes it sound that way.

Also if I want to use a blue or white LED instead of the red, can I just
change out the resistor to get the correct current going to the higher
voltage LED?

You can see the schematic here...

http://hp15c.org/BL1.gif

Q1 and Q2 are 2N3904
R1 and R2 are 100 ohms
R3 and R4 are 47k ohms
C1 and C2 are 4.7uf
If you look at the hfe of the 2N3904 which = 100. Ic should be = Ib * hfe =
Click to expand...
32ma at 15 volts and 5ma at 3volts.
Maybe they're constant-current LEDs? Otherwise, there's no reason why the
circuit will maintain a constant current regardless of voltage.
Click to expand...
 
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